Differences in smooth pursuit characteristics in different types of strabismus.

Smooth pursuit is a type of eye movement that allows us to maintain a moving target near the fovea. Although the main characteristics of smooth pursuit are well described, even for young people, not many studies have analyzed performance in the case of strabismic patients. Furthermore, it is not clear if there are differences between the different types of strabismus. In this study, we analyzed smooth pursuit eye movements of young strabismic patients suffering from intermittent exotropia or accommodative esotropia, comparing performance with that of controls. Participants were 54 children, of ages between 4 and 13 years old (21 cases of accommodative esotropia, 17 cases of intermittent exotropia and 16 controls). Eye movements were recorded while performing different tasks. To produce smooth pursuit a single image was presented at the center of the screen, which jumped to a new position 14º to the right or to the left and immediately started to move horizontally across the screen with a sinusoidal movement, with a velocity of 11º/s. Smooth pursuit latency and gain, as well as the number of saccades during pursuit, were analyzed. Smooth pursuit showed different characteristics in the three groups, although variability was very high. Gain was significantly lower in intermittent exotropia, compared to the control group. In contrast, for smooth pursuit latency only patients with esotropia differed from controls. Accomodative esotropia patients also showed a significantly higher number of saccades during pursuit, and of a larger amplitude. Main differences with the other groups were also found in anticipatory saccades. When all variables were analyzed according to age, we found that each group progressed differently during development. These results show that smooth pursuit is affected by strabismus, but performance differs between clinical subtypes. More studies are needed to understand these developmental effects and evaluate clinical treatment outcomes.